JP 2003-224902 A discloses a method of diagnosing performance of capacitors for a vehicle in an electric motor system for a vehicle which utilizes energy accumulated in capacitors provided on the vehicle, wherein charging current for the capacitors for a vehicle is measured based on voltage values of precise resistances, and capacitances or internal resistances of the capacitors are measured based on charging current after a startup of the system and initial changes of the voltages of the capacitors.
JP 2006-126172 A discloses a method of detecting a status of a secondary battery comprising: a step of obtaining data of pairs of electric currents passing through the secondary battery and a terminal voltage of the secondary battery associated with the electric current; a step of performing statistical processing for the data of the pairs effective at the time of charging and discharging to calculate an internal resistance value at the time of charging and discharging of the secondary battery based on a gradient of an approximate line obtained by the statistical processing; a step of performing statistical processing for the data of the pairs effective at the time of charging to calculate an internal resistance value at the time of charging of the secondary battery based on a gradient of an approximate line obtained by the statistical processing; and a step of calculating the internal resistance value of the secondary battery based on the internal resistance value at the time of charging and discharging and the internal resistance value at the time of charging.
In order to precisely measure an internal resistance of an electric power accumulator, it is desired to use electric current values and voltage values before and after a change from a state in which there is substantially no charging or discharging in the electric power accumulator to a state in which the charging or the discharging is performed.